Turbochargers are a type of forced induction system. They deliver compressed air to the engine intake, allowing more fuel to be combusted, thus boosting the engine's horsepower without significantly increasing engine weight. This can allow for the use of a smaller turbocharged engine, replacing a normally aspirated engine of a larger physical size, thus reducing the mass and aerodynamic frontal area of the vehicle. Turbochargers use the exhaust flow from the engine to drive a turbine, which is mechanically connected to a compressor. At startup, the turbocharger may be at temperatures well below 0° C. Since the turbine spins at extremely high speed, in the range of 150,000 RPM to 300,000 RPM, is mechanically connected to the exhaust system, it sees high levels of temperature, up to 1050° C. for a gasoline engine, and vibration. Such conditions have a detrimental effect on the components of the turbocharger. Because of these adverse conditions the design, materials and tolerances must be selected to provide adequate life of the assembly. The design selections, required to satisfy these conditions, often lead to larger than preferred clearances, which, in turn, cause aerodynamic inefficiencies. Further, the flow of exhaust gasses impart rotational torque on the vane assembly, which must be prevented from rotation by mechanical securing means.
Turbochargers, which utilize some form of turbine flow and pressure control are called by several names and offer control though various means. Some have rotating vanes, some have sliding sections or rings. Some titles for these devices are: Variable turbine design (VTG), Variable geometry turbine (VGT) variable nozzle turbine (VNT), or simply variable geometry (VG). The subject of this patent is the rotating vane type of variable turbine, which will be referred to as VTG for the remainder of this discussion.
VTG turbochargers utilize adjustable guide vanes (FIG. 1 (80)), rotatably connected to a pair of vane rings (30, 20) and/or nozzle wall. These vanes are adjusted to control the exhaust gas back pressure and the turbocharger speed by modulating the exhaust gas flow to the turbine wheel. The vanes are rotatably driven by the fingers (FIG. 7, 61), which are located above (in the direction of assembly, i.e., to the left in FIG. 1) the upper vane ring (30). For the sake of clarity, these details have been omitted from most of the drawings. VTG turbochargers have a large number of components which must be assembled and positioned in the turbine housing so that the guide vanes remain properly positioned with respect to the exhaust supply flow channel, and the turbine wheel, over the range of thermal operating conditions to which they are exposed. Typical VTG turbochargers employ three fasteners (111, 112, 113) which are either studs, bolts, or studs with nuts, to secure the vane ring assembly (e.g., the vane ring and guide vanes) to the turbine housing (100) so that the turbine housing assembly surrounds the vane ring assembly. The fasteners pass through both vane rings to clamp the upper vane ring to the spacer, the spacer to the lower vane ring, and the lower vane ring to the turbine housing.
The connection of such an assembly to the turbine housing produces several important issues: As can be seen in FIG. 7, the parallelism of the vane ring assembly including vane rings (20) and (30) must be parallel to the turbine housing (100). The vanes (80) must be placed such that the vane cheek surfaces (81) are adjacent to and parallel to the upper and lower vanes rings. The turbine housing machined face (101) must be machined in the correct axial location for the vanes to line up with the turbine flow at the entry of the turbine wheel. The angular location of the vane ring assembly to the turbine housing datum (FIG. 9, 126) is determined by the radius from the centerline of the bore of the turbine housing and a set of coordinate dimensions (124). These dimensions determine the X-Y-Z location of the vane assembly to the turbine housing.
The effect of temperature on the turbine housing results in both thermal expansion (at the rate of the coefficient of thermal expansion for the iron or steel of the turbine housing or respective part being heated) influenced by the thermal flux caused by the flow path of the exhaust gas, which is additionally influenced by the geometry and wall thickness of the turbine housing. The inherent nature of a turbine housing under thermal influence is for the “snail section” to try to unwind from its ambient temperature shape and position. This often results in a twisting motion, dependant upon the constraints of the casting geometry. Unconstrained by attachment to the turbine foot, gussets or ribs, the turbine housing large apertures, which are cylindrical at room temperature, assume an oval shape at operating temperature.
This relatively simple thermal expansion, combined with the results of the geometric and thermal flux influences, results in complex deformation of the turbine housing across the temperature range.
When an assembly, such as the vane ring assembly, is mounted to the turbine housing wall as in FIG. 1 and FIG. 4, the studs or bolts (8, 13) will assume the motion of said wall, albeit in a manner somewhat perpendicular to said wall. When the turbine housing wall moves due to thermal influences, the mountings will mimic that movement. In FIG. 10, the fasteners (111), (112), (113) are each held in perpendicular position by the tapped holes (121), (122), (123) in the turbine housing (100).
The fasteners (111), (112), (113) are held in both X-Y and angular position by the placement of the tapped holes in the turbine housing. The relative position of each hole, to the center of the turbine housing (120), is determined by the coordinate X-Y positions of each tapped hole (121), (122), (123) to the coordinate position of the turbine housing center (120), and the angular position by the relationship of the set of the three holes to a datum (126), determined by the X and Y coordinates (124) (see FIG. 9).
FIG. 11 shows that a simple case of distortion in the turbine housing mounting face (101) has a large effect, offset, but basically perpendicular to the turbine housing mounting face as in FIG. 10. The base position of the fasteners (111, 112, 113), determined by the tapped holes (121, 122, 123) in the turbine housing, on pitch circle diameter (PCD) (125, FIG. 9), changes a small amount due to the change from flat to curved of the turbine housing mounting face (101). It can be seen however in FIG. 11 that the dimension “A” at top end of the fasteners (111), (112), 113) moves considerably more, than does the dimension “B” at the bottom end of the fastener. The angular position of the fasteners, relative to the datum (126) stays relatively constant. In a like manner the distortion of the turbine housing could be convex, instead of concave, which would result in the dimension, at the top end of the fasteners, moving in a direction which produces a top end dimension “A” being less than the bottom end dimension “B”. The important thing is the deformation and motion, not the direction of deformation, and resultant motion. This is a simple case of distortion, which does not take into account the planar change in tapped hole position due to simple thermal coefficient of expansion. In this case, which overlays the above, the circular machined bores become an oval shape, which further exacerbates the situation.
This displacement of the fastener causes distortion in the vane rings, which then causes the vanes and moving components to jam. If the clearances between components are loosened in order to reduce sticking of the vanes, the added buffer clearances cause a loss of aerodynamic efficiency, which is unacceptable. The clearance between vane side faces (FIG. 12 (81)), and their partner vane ring inner faces is especially critical to aerodynamic efficiency. The displacement of the fasteners also generates high stress in the fastener, which results often in failure of the fastener. Unusual wear patterns, due to distortion in the vane ring, also generate unwanted clearances, which further reduce the aerodynamic efficiency.
Tapped holes are a reasonably efficient manufacturing method but are simply not effective when it comes to dimensional accuracy or repeatability. While it is normal practice to generate acceptable accuracy and repeatability with drilled or reamed holes, the threading activity is fraught with problems. The threaded region of both the fastener and the hole has to be concentric with the unthreaded zone of the shaft and hole in order to place the fastener in the appropriate X-Y position with respect to the hole. By the very nature of threads it is usual for the male feature to lose its perpendicularity to the female feature (and vice versa) as increased torque applied to the fastener rocks the un-torqued portion of the fastener towards the thread angle, which has the effect of tipping the fastener, in the case of a male stud or bolt in a female hole, away from perpendicular to the threaded surface plane.
In U.S. Pat. No. 6,558,117 to Fukaya, a VTG turbocharger is shown having a vane ring assembly integrally connected to the turbine housing via bolts. The Fukaya device is shown in FIGS. 2, 3 and 4, has a turbine casing (1), rotatable guide vanes (2), a flow passage spacer (3), a bill-like projection portion (4) and a turbine rotor (5). Each of the guide vanes (2) is supported by a rotational shaft (7) extending outward of a guide vane table (6). A bolt (8) extends through the guide vane table (6) and the flow passage spacer (3), and is fastened to the casing (1).
To account for thermal deformation of the casing (1) and the guide vane table (6), an outer diameter of the Fukaya flow passage spacer (3) must be set to about 9 mm. Fukaya also uses material selection to combat thermal expansion. A material having the same coefficient of linear expansion as that of the guide vanes (2) (for example, SCH22 (JIS standard)) is employed for a material of the flow passage spacer (3) and the bolt (8). A width hs of the flow passage spacer (3) is designed to be slightly larger than a width hn of the guide vanes (2), and an attempt is made to minimize the gap between both of the side walls of the casing (1) and the guide vane table (6) sectioning the turbine chamber, and the guide vanes (2).
Due to the integral connection of the housing (1) with the vane table (6), the Fukaya turbocharger suffers from the drawbacks of having to allow clearances to account for thermal growth. Such gaps reduce the performance of the turbocharger. The Fukaya turbocharger also requires the use of material with a low thermal coefficients of expansion. Such materials can be costly and difficult to work with.
Fukaya further proposes another variable geometry turbocharger as shown in FIGS. 3 and 4. Three bolts (13) each having an outer diameter of 5 mm are arranged at positions uniformly separated into three portions in a peripheral direction. The bolt (13) extends through a portion of the guide vane table (6) that extended to the casing (1) side and fastens the guide vane table (6) to the casing (1). A heat-resisting cast steel HK40 (ATSM standard) having a little amount of carbon is employed for a material of the casing (1), the guide vane table (6) and the guide vane (2). A distance between both of the side walls of the casing (1) and the guide vane table (6) is defined by ha−hb, and is designed to be slightly larger than the width hn of the guide vane (2).
While this other embodiment of Fukaya removes the fasteners from the flow path, it still provides an integral connection of the housing (1) with the vane table (6), which will result in the transfer of stresses and/or growth from the casing to the vane ring components. The Fukaya turbocharger also requires the use of material with low thermal coefficients of expansion. Such materials can be costly and difficult to work with.
In U.S. Pat. No. 6,679,057 to Arnold, a variable turbine and variable compressor geometry turbocharger is described as shown in FIG. 5. Each of the turbine vanes is connected to the turbine housing via a vane post. The vane post is inserted into a correspondingly sized hole in the turbine housing. The Arnold device also suffers from the drawback of radial thermal expansion of the turbine housing imparting undue stress and/or movable components “sticking” due to the use of the vane post connection in the housing.
In U.S. Pat. No. 7,021,057 B2 to Sumser, an exhaust-gas turbocharger with a VTG vane structure is described as shown in FIG. 6 in which spacer bushes (21) are provided to ensure that there is a defined minimum distance between the outer support wall (11) and the inner support wall (14). The variable turbine vane structure is fixed by means of bolts (22), which extend between the end section (17) of the support wall (14) and the support wall (11). Also here, the vane ring components will suffer thermal stresses imparted by the turbine housing due to the fixed structure.
U.S. Pat. No. 5,186,006 to Petty, references cross cut keys as a method for the mounting of a ceramic shell defining a turbine housing onto a metal engine block using a set of ceramic cross cut keys connected to a second set of cross cut keys on a metal spider bolted to the engine block.
U.S. Pat. No. 6,287,091 to Svihla et al, references radial keys and guides to be used in aligning the nozzle ring of an axial turbocharger for a railway locomotive.
FIG. 20 depicts the centering drive from a Cosworth DFV, or DFX racing engine. These engines were first produced in 1967 and have been in general production for some 40 years. This drive mechanism is used to provide drive to the oil and water pumps on the sides of the engine, irrespective of the thermal conditions of either pump. The temperature of the fluids in the pumps cause the pumps to expand or contract against the engine block, thus changing the centerlines of the pumps, relative to the driving flange which is also solidly mounted to the engine block, albeit under a different set of thermal conditions. So in most cases the center of the flanges is not concentric with its mating flange, but the design enables a vibration free drive to take place.
In this design the driving flange (182) is screwed onto a driving shaft (187) connected by belt drive to the engine crankshaft. The driving flange features a radial male key (186), which engages into a female radial slot (185) in the cross-key coupler (180). In this embodiment of the cross-key design, the coupler (180) has two diametral keys, one male (185) and one female (184) at an angle of 90° to each other. The driven flange (181) features a male key (183) machined into its face. The male key engages in the female slot (184) in the coupler (180). The coupler is held in axial position only by the proximity of the driving, and driven, flanges. The coupler is held in radial position by the action of the two mating keys and keyways in the opposing flanges. Thus the coupler provides a centerline drive from the driving flange (182) to the driven flange (181).
Thus, there is a need for a fastening system and method for connecting the vane ring assembly to the turbine housing. There is a further need for such a system and method that accounts for thermal growth and distortion of the turbine housing and/or vane ring assembly while maintaining peak efficiency. There is a yet a further need for such a system and method that is cost effective and dependable. There is additionally a need for such a system and method that facilitates manufacture, assembly and/or disassembly.